1. Technical Field
The present invention relates generally to distributed generation systems. More specifically, the present invention provides an inverter control methodology for maximum power point tracking (MPPT), anti-islanding, and output current regulation for distributed generation sources connected to a utility grid.
2. Related Art
The available power output from a solar photovoltaic (PV) array (comprised of one or more solar photovoltaic panels configured in some combination of parallel and series) depends on the amount of sunlight hitting the array (the insolation) and the array temperature. The output power is the product of the array voltage and array current. It is a characteristic of solar PV arrays, as well as other distributed energy sources, that the output voltage drops as the output current increases. With solar PV arrays, the output voltage drops monotonically with increasing output current. At some level of output current, the output voltage will begin to drop much more rapidly. Once this occurs, the power being output by the solar PV array begins to drop.
FIG. 1 shows a qualitative plot of array output voltage versus array output current for one level of insolation and one array temperature. Also shown in FIG. 1 is the power output by the solar PV array, indicating that the peak power delivered by the array occurs when the solar PV array is operating at the “knee” of the voltage-current characteristic. The voltage and power curves given in FIG. 1 change for other levels of insolation and array temperature, though they always maintain similar shapes. It is a standard objective to preferentially load the solar PV array so that it is operated at the maximum power point. Because of dynamic changes in array temperature and insolation, the tracking of the maximum power point is a dynamic process. Further, the double-valued nature of the cell power as a function of array current demands resolution of which side of the maximum power point the inverter is on in order to move in the correct direction toward the point of maximum power.
Maximum power point tracking (MPPT) under variable conditions (i.e. changing light intensity, changing temperature, and different solar photovoltaic characteristics) has been proposed by U.S. Pat. No. 4,404,472 (Steigerwald), U.S. Pat. No. 4,375,662 (Baker), and U.S. Pat. No. 5,268,832 (Kandatsu), incorporated herein by reference. Steigerwald proposes the perturb and observe method for detecting the maximum power point (MPP) to handle rapid changes in insolation. The commanded value of array current (the output current gain) is compared to the actual array current, and a decision is made to increase or decrease the command value. The magnitude of the output power change is compared with the expected change in magnitude. The current command is then changed in the direction associated with an increase in the array power. Baker proposes measuring the voltage and current from the array, so as to keep the slope of the voltage versus current curve at unity magnitude by changing the output power. The methodology uses a reference value that is determined by superimposing a current load on the array. The change in array voltage is used as an indication of the present operating point on the voltage versus current slope. Kandatsu proposes a method that forces the output power variation nearly to zero so as to maximize the average output power around the MPP. This method is used when the rate of change of the insolation is small; otherwise a method similar to the MPP method used by Steigerwald becomes active during rapid changes.
Kasa et al., “Robust Control for Maximum Power Point Tracking Photovoltaic Power System,” Proceedings of the IEEE Power Conversion Conf., pp. 827-832, September 2002, proposes voltage control via the perturb and observe method and using an extra controller to maintain stability in the face of loose tolerances in array (solar PV array) parallel capacitance. Yaoqin et al., “A New Maximum Power Point Tracking Control Scheme for Wind Generation,” Proceedings of the IEEE Power System Technology Conf. (PowerCon), pp. 144-148, February 2002, also uses the perturb and observe method as applied specifically to wind turbines. Pan et al., “A Fast Maximum Power Point Tracker for Photovoltaic Power Systems,” Proceedings of the IEEE Industrial Electronics Conf. (IECON), pp. 390-393, March 1999, uses a different method by controlling the array current by a continuous analog controller. Each of the above-mentioned articles is incorporated herein by reference.
Another issue with utility interactive inverters is the detection of islanding. An islanding condition is created when the utility supply is interrupted but the inverter continues to provide energy to the utility system. If such a condition were allowed to persist, utility personnel are put at risk if they should come in contact with the section of the utility system that is energized by the inverter. For this reason, it is a safety requirement that any inverter that interacts with the utility be able to detect when the utility becomes de-energized. When such a condition is detected, the inverter must disconnect itself from the utility within a fixed amount of time. There are various ways to detect an islanding condition, but the most common approach is the one promulgated by Sandia National Laboratory. With this approach a small amount of positive feedback is used to modify the inverter output current amplitude and frequency based on the natural variations of the utility voltage and frequency. In this way the inverter is always trying to push the utility toward conditions that will cause the inverter to disconnect itself. This method is effective if the utility grid is sufficiently strong to resist the destabilizing actions of the inverter.
Methods to prevent islanding using voltage or frequency deviations has been proposed by U.S. Pat. No. 6,219,623 (Wills), incorporated herein by reference. Wills proposes: when a number of deviations have been detected over a given period of time, the control tries to force the output voltage or frequency in an accelerating manner outside the limits of normal grid operation to trigger a fault condition to shut down. This is similar in nature to the anti-islanding algorithm developed by Sandia National Laboratory. The essence of this algorithm is the use of a small amount of positive feedback in the inverter control so that increases in line frequency cause the inverter to try and increase frequency still further. Voltage variations are handled in a similar manner.
A third issue of importance to the performance of a utility-interactive inverter is the control of the inverter output current waveform. The general objective is to force the inverter output current to be of the same shape as the utility voltage and in phase with the utility voltage. That is, the output current waveform should be of low distortion.
There is a need, therefore, for an improved inverter control methodology for maximum power point tracking (MPPT), anti-islanding, and output current regulation for distributed generation sources connected to a utility grid.